The present invention relates to a method and a mold for injection molding of thermoplastic material articles, especially polycarbonate (PC) articles and the use of such a method and mold to obtain injection molded optical lenses and in particular minus lenses free of weld lines.
Typically, thermoplastic optical lenses are made by injection molding of a thermoplastic material in a molding cavity defined by the internal surface of a two-piece mold insert.
In injection molding of thermoplastic materials, weld lines are formed whenever advancing melt fronts meet, which may originate from multiple gate filling to leading to head-on impingement of separate flow fronts or splitting and converging of flow fronts due to presence of insert. Weld lines are important not only because they form visual defects such as lines, notches or color changes on the surface of the molded part, but also because they generally represent weak regions due to the difference in molecular orientation across the weld regions.
In the case of injection molding of optical lenses, a thermoplastic material melt, at a temperature above the glass transition temperature (Tg) of the thermoplastic material, is injected in the mold cavity. The flow fronts then wrap around the edge of the mold cavity and then rejoin at the end of the cavity, forming a weld line. This problem of weld line formation occurs predominantly when molding minus lenses.
The strength of the weld line may be characterized by a weld line strength factor, which is defined as the ratio of the composite strength in the presence of a weld line to that of the same composite without weld lines.
Generally speaking, the weld line strength factor can be enhanced by increasing injection pressure, speed, and melt temperature or by locating weld lines closer to a gate to get enough packing. Also, providing venting at the weld region helps as the resistance to packing flow is reduced. However, none of these practices can eliminate weld lines completely. In ophthalmic lens application, presence of weld lines is simply unacceptable due to the resulting cosmetic defects and sometimes power variations.
Several kinds of mold inserts are known in the art for molding optical lenses.
One kind of mold insert which is used is a glass mold insert.
Other kinds of mold inserts are also described in the prior art.
Thus, U.S. Pat. No. 4,793,953 describes a die comprising a base material of Beryllium-Copper alloy having high heat transfer rate with a thick plating film of chromium or nickel giving desirable surface properties. The film is however very thin (up to about 380 micrometers). In this document it is indicated that optical mold made of electroformed metal (usually nickel) of thickness up to about 0.4 cm are known in the art and are preferably used instead of glass molds because their surfaces have an increased transfer rate, which is indicated as desirable for use in high pressure thermoplastic molding processes. It is also mentioned, that there have been previous attempts to combine, in a two-piece mold insert, metal materials of dissimilarly high and low thermal conductivities, to produce optical lens injection molds for thermoplastic materials. Reference is made to U.S. Pat. No. 2,292,917 which discloses a two piece mold insert with a steel die joined with a back body formed of a material having a thermal conductivity considerably higher than that of the material of the dies.
U.S. Pat. No. 4,364,378 describes a two-piece mold insert with a conductive plug made of a material of lower thermal conductivity than the remainder of the insert and allowing obtention of a higher heat transfer rate at the peripheral portion of the mold than in the center of the mold.
Document JP 07266343 describes the use of a metal mold with an inner surface coated with a thermal insulation layer of a polymer having a thermal conductivity of 0.002 cal/cm.s. xc2x0 C., and having a thickness of 0.01 to 2 mm, this thermal insulation layer having fine rugged surface. Due to the fine rugged surface, this mold is not suitable for molding optical lenses.
Nevertheless, the weld line formation problem still remains when using these prior art molds.
U.S. Pat. No. 4,364,878 describes an apparatus and a method for molding precisely shaped articles such as ophthalmic lenses, and in particular minus lenses, from synthetic resin. In order to solve the weld line problem, it is proposed in U.S. Pat. No. 4,364,878 a method which comprises moving the insert pieces to their relatively remote position to form a cavity of a volume greater than that of the finished lens, injecting into the cavity at a relatively low pressure a parison mass of the resin equal to the mass required to form the finished product and applying pressure to the insert pieces to bring their optical surfaces into engagement with the mass to coin the mass to fill completely the space between the insert pieces, and controllably cooling the viscous mass while under pressure so that all parts thereof cool to the glass transition temperature at substantially the same time. This process, which necessitates moving the insert pieces is complex and requires a sophisticated molding machine.
U.S. Pat. No. 4,560,342 also discloses a method for molding weld line free concave lenses using a metallic insert. The above goal is reached according to U.S. Pat. No. 4,560,342 by firstly feeding the molten resin material through a feeding mouth into an intermediate thin portion of the mold cavity, while permitting hot water to circulate to maintain a high mold temperature and thereafter moving the feeding mouth to a thick portion of the mold cavity, and continuously feeding with molten resin. The implementation of such a method necessitates a complex device, in particular for moving the molten resin feeding mouth.
One object of the present invention is to provide a method and mold for molding thermoplastic articles, such as ophthalmic lenses and in particular minus ophthalmic lenses, free of weld line.
An other object of the present invention is to provide a method and a mold as defined above which result in shorter molding cycles.
In general, the invention contemplates a method for injection molding a weld line free thermoplastic material article which comprises:
injecting a melt of a thermoplastic material at a temperature higher than the glass transition temperature (Tg) of the thermoplastic material in a molding cavity defined by a two piece mold insert, and
after cooling and disassembling of the two piece mold insert, recovering the molded article,
wherein each of the two-pieces of the mold insert comprises at least one portion partly defining said mold cavity which is made of a material or a combination of materials, other than glass the thermal diffusivity xcex1 of which fulfills the condition
1 less than xcex1/xcex1g less than 11 
where xcex1g is the thermal diffusivity of a borosilicate crown glass and is equal to 6.20E-7 m2 sxe2x88x921, said portion having a thickness of at least 1 mm.
The invention also contemplates a two-piece mold insert for use in a mold for molding of a weld line free thermoplastic material article, wherein at least a portion of said insert is as defined above in connection with the injection molding method.
Preferably, the material constituting the portion of the pieces of the insert is a metallic material.